Our dogs and cats typically live only one-sixth as long as we do but often receive five times the number of vaccines, according to Justine Lee, DVM. This has been due in part to the outdated idea that every annual shot was absolutely necessary. But in 2006, two top veterinary associations updated immunization guidelines: "We're now taking into account the needs of individual pets, rather than using the one-schedule-fits-all approach of the past," says Link V. Welborn, DVM, who chaired the American Animal Hospital Association's canine protocol update.

One reason: While it's difficult to directly link specific health problems to vaccines, a growing body of research suggests that too many shots may play a role in a range of maladies--everything from immediate allergic reaction to seizures and arthritis--and even cancerous tumors at injection sites years later.

That's not to say that vaccines aren't necessary. They've long protected our pets from debilitating diseases and untimely death. Puppies and kittens, especially, need a series of shots to make up for the waning of antibodies passed on at birth by the mom. Vets--even holistic vets who tend to avoid vaccines--stress that this series is crucial.

If you're an owner who wants to protect a loved one from both disease and the risks of over-immunization, read on.

KNOW THE LAW

Always check your city or county's Web site to learn which vaccinations are required by the local department of animal control. Most insist only on rabies, though additional vaccines may be mandated for pooches that frequent dog parks. Some municipalities accept 3-year shots, while others demand annuals. Additionally, some will allow titers (see below) or an exemption letter from a vet, stating that your pet may be at risk if vaccinated. But aside from what laws dictate, the decision about what shots to give and when is entirely up to you and your vet.

WHEN A TEST IS BEST

Titers are blood tests vets use to determine whether or not antibodies from previous vaccinations are still protecting your animal. A test typically costs around $50, depending on your location. If results show adequate antibody levels, your pet can skip her shot, no matter how long it's been since her last; if levels are inadequate, head back to the vet for a booster.

SKIP COMBO SHOTS

Dogs are up to 24% more likely to have a bad reaction when two or more vaccines are given at a time, according to one study. And always give rabies in a separate visit, cautions Jean Dodds, DVM, a member of the board of the American Holistic Veterinary Medical Association. Check with your vet--many will waive the cost of a second visit, so you pay the same as having all shots at once.

ASSESS THE RISK

Factors such as age, sex, and breed of your pet, as well as lifestyle, play a large role in her potential reaction to a vaccine. Purebred animals may be at an increased risk, and especially in dogs, size makes a difference--small breeds are more prone to a reaction than large. (Unlike with other drugs, vaccine dosage is the same for a 7-pound Yorkie and a 100-pound Great Dane.) And once a dog reaches age 6 or 7, it's likely that she's built up enough antibodies to keep her safe from the most common viral infections for years. In these cases, ask for titers and opt for 3-year boosters, whenever possible.

It's also crucial to know how your pet spends her days. A dog that frolics in the Berkshire woods, for instance, may require the Lyme disease vaccine, while a pup kept on a leash in Las Vegas doesn't need it. Leptospirosis--a bacterial disease transmitted by water--isn't a risk in some states but is a danger in others. And a solo indoor cat can forgo some boosters. Your vet can help you develop the right plan.

What You Need To Know

Here, a rundown of recommended shots from the American Association of Feline Practitioners (aafponline.org) and the American Animal Hospital Association (aaha net.org). Go to these sites for the full list.

CORE: Necessary for all pets because the diseases they prevent are threats to either pet or public health. Rabies parvovirus (intestine), distemper (respiratory/nervous system), adenovirus (liver/respiratory) Rabies, panleukopenia virus (FPV) (intestine), herpesvirus-1 and feline calicivirus (FHV-1/FCV) (respiratory)

OPTIONAL: Safe and effective but may not be essential, depending on your pet's lifestyle. Parainfluenza (respiratory), bordetella (respiratory), leptospira (liver/kidney) Feline leukemia virus (FeLV) (cancer), feline immunodeficiency virus (FIV) (immune system), bordetella (respiratory)

NOT NECESSARY: Immunizations that vets have deemed ineffective or for which the risks of the diseases are low.
Coronavirus (intestinal) Feline infectious peritonitis (FIP) (blood-borne, affects organs)